1. Field of the Invention
The present invention relates to a gear tooth surface modification, and more in particular to a technique for suppressing variation in working transmission error owing to working torque difference such that the working transmission error can be reduced in a wide working torque range.
2. Description of the Related Art
The "tip relief" and the "bias-in" are widely known as the techniques for reducing the transmission error that is exciting force to cause gear noise, as described in "Collection of Articles, Vol. 40, No. 340, p.3514", The Japan Society of Mechanical Engineers, and "MPT 91 Collection of Articles", p.92, respectively. The tip relief technique (lacking tooth tip portion not as not to contact a tooth of a mating gear) is intended to cancel variation caused by working rigidity of the teeth by adding modification amount to each tip of the teeth (corresponding to the starting/end working point of the helical gear) of the driving and driven gears, thereby minimizing the exciting force at a target torque. Meanwhile the bias-in technique (shape of tooth width is gradually charged, and contacting point becomes longer along with working direction) is intended to utilize the gear specifications, especially the contact ratio, effectively by continuously changing the pressure angle error across the tooth width such that each gear tooth is kept in contact with its mating tooth from the starting working point to the end working point for a time as long as possible. In this technique, the more the number of gear teeth simultaneously in contact increases, the less the adverse effect of each transmission error becomes. The working transmission error as a rotational error of a pair of gears meshed with each other may be expressed, for example, by a maximum lead/lag (rad) of a driven gear when a driving gear is rotated at a predetermined rate.
In the above-mentioned general technique of gear tooth surface modification, however, the torque-dependent characteristic of the transmission error may cause the problem as described below. That is, the gear noises can be reduced under predetermined operating conditions but are not always sufficiently reduced under other operating conditions where the working torque (load torque) is different. According to the above-described technique, the gear profile is modified on the assumption that the working torque is kept constant. Accordingly when the working torque changes, the resultant elastic deformation of supporting members or flexural deformation of the tooth itself changes the position of the tooth surface. As the locus of contact points changes when a position of the tooth surface changes, a target characteristics cannot be obtained. The locus of contact points is a line defined by plotting points on the tooth surface where gear teeth come into contact with each other earliest at each rotational position in conjunction with the gear rotation.